If you have ever worn a black t-shirt on a sunny day, you know firsthand that dark materials absorb more heat from the sun. Dark surfaces in the sun can become up to 70°F hotter than the most reflective white surfaces. If those dark surfaces are roofs, some of the heat collected by the roof will be transferred inside. The ability for building occupants to stay comfortable inside a building with a dark coated roof means more air conditioning and higher utility costs. These dark roofs also heat the area around them, contributing to the heat island effect.

We know that trees cool buildings and people by providing shade, and remove heat from the air by evaporating water. Solar-reflective roofs and pavements use materials that reflect much of the sun's radiation back to the sky, keeping their surfaces cool and reducing the flow of heat into the air. White roofs are coolest, but dark roofs can also be made to reflect the invisible "near-infrared" component of sunlight. These white or colored "cool" roofs reduce the need for cooling energy in air-conditioned buildings and make unconditioned buildings more comfortable.

Although the majority of reflective roofs installed today are white in color, with advanced work in infrared-reflective pigments, coatings manufacturers have been able to incorporate these types of color pigments into their high-performance exterior coatings. These coatings, such as field-applied, air-dried fluoropolymers, can be used on architectural metal roofing. With this new technology, the architectural community now has the ability to obtain a cool, sustainable energy-saving roof without having to sacrifice color and aesthetics.

Regardless of their material, cool roofs have two important surface properties: a high solar reflectance and a high thermal emittance.

Solar reflectance is a measure of the ability of a surface material to reflect sunlight. This includes the visible, infrared and ultraviolet wavelengths – measured on a scale of 0 to 1. Essentially, it is the percentage of solar energy that is reflected by a surface. Solar reflectance is also called "albedo."

Thermal emittance is defined as the percentage of energy a material can radiate away after it is absorbed. The Solar emittance of a material refers to its ability to release absorbed heat. Scientists use a number between 0 and 1, or 0% and 100%, to express emittance. With the exception of metals, most construction materials have emittances above 0.85 or 85%.

Energy Star and Leadership in Energy and Environmental Design (LEED) both have adopted requirements for the Solar Reflectance Index and Emisivity of roofing materials. LEED’s most widely known credits for roofing are 7.1 Heat Island Effect (Non-Roof) and 7.2 (Roofs). According to LEED, the intent of these credits is to 'reduce heat islands' characterized as thermal gradient differences between developed and undeveloped areas. It is hoped this will minimize impact on microclimate and human and wildlife habitat.

According to the U.S. Green Building Council, Steep-Slope Solar Reflectance Index (SRI) is defined as a measure of the constructed surface’s ability to reflect solar heat as shown by a small temperature rise. It is defined so that a standard black with reflectance of 0.05 and emittance of 0.90 is zero, while the SRI for a standard white with reflectance of 0.80 and emittance of 0.90 is 100. To calculate the SRI for a given material, it is necessary to obtain the reflectance and emittance values for the material. SRI is calculated according to ASTM E 1980, a standard practice for calculating the solar reflectance index of horizontal and low-sloped opaque surfaces with emissivity greater than 0.1. Reflectance is calculated according to any one of three ASTM standards—ASTM E 903, ASTM E 1918 or ASTM C 1549.

LEED Version 3 requires that roofing materials used on low-slope roofs with a slope of less than 2:12 must meet a solar reflectance index, or SRI, of 78. Steep-sloped roofs with a slope of more than 2:12 must have an SRI of 29.

Tnemec’s line of fluoropolymer topcoats, HydroFlon and Fluoronar, contain infrared-reflective pigments which help reflect the sun's natural light, and therefore help to keep the structures they are protecting, cool inside and out. Tnemec offers an array of light, warm and deeptone colors that meet the SRI guidelines outlined in LEED Version 3 (v3). For more information on this subject matter, or to learn more about fluoropolymer technology, please contact your local Tnemec representative at www.tnemec.com. 

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